1. Field of the Invention
The present invention relates generally to the field of graft polymerization and, more specifically, to graft polymerization of non-woven polyolefin fiber sheets, and to batteries in which graft polymerized non-woven polyolefin fiber sheets constitute separators which are positioned between the positive and negative plates.
2. Description of the Prior Art
Battery separators which are non-woven sheets of polyolefin fibers with an acrylic graft polymer on the fiber surfaces are known, being disclosed in a paper presented at the 1986 Power Sources Symposium held in Cherry Hill, N.J., J. Polym Sci. 34, 671 (1959), and in WO 93/01622, published Jan. 21, 1993. The Symposium paper discloses the use of ionizing radiation to produce such separators, while WO 93/01622 discloses a method for producing such separators which involves immersion of non-woven sheets of polyolefin fibers in a solution of an acrylic monomer which also contains a photoinitiator, followed by ultraviolet irradiation of the sheets. Various publications, e.g., Yao and R{dot over (a)}nby, Journal of Applied Polymer Science, Vol 41, 1469-1478 (1990) disclose work carried out at the Royal Institute of Science, Stockholm, Sweden, which involved immersing polyolefin fibers and films in solutions of acrylic monomers and benzophenone, followed by ultraviolet irradiation of the fibers and films to produce an acrylic graft polymer on the surfaces of the fibers and films. Another publication, Journal of Polymer Science, Polymer Letters Edition, Vol. 19, pages 457-462 (1981) discloses the immersion of polyolefin films in an acetone solution of benzophenone and, after drying of the films, vapor phase and liquid phase copolymerization of an acrylic monomer with the polypropylene surfaces of the films to produce acrylic graft polymers.
A Journal article entitled “Chemical Absorptive Properties of Acrylic Acid Grafted Non-woven Battery Separators”, refers to work carried out by Leblanc et al. (citing P. Leblanc, Ph. Blanchard, S. Senyarich, Abstract No. 261 ESC/ISE meeting, Paris, (1997) and P. Leblanc, Ph. Blanchard, S. Senyarich, Electrochem Soc. 145, 846, (1998)) as showing that ammonia in a NiMH cell dramatically reduces the self-discharge performance, and cites the latter reference for the statements:                “It was also demonstrated that this effect could be significantly reduced if the free ammonia in the cell could be removed. The tests showed that by using an acrylic acid grafted separator with an ammonia absorption of over 1.5×10−4 mol/g then all the free ammonia could be eliminated, and the self-discharge performance improved to the levels normally associated with NiCd cells.”The reference then refers to the process of WO 93/01622 as “capable of grafting non-wovens of all constructions” and states:        “A study was carried out to examine the effect of the non-woven type on the separator's ability to absorb ammonia using Kjeldhal's technique (see table 1). All the grafted materials were grafted to the same level using the same grafting conditions. The results show firstly that an acrylic acid graft is necessary for a non-woven to posses the ability to absorb ammonia. Furthermore, the amount of absorption is a function of the base non-woven material. The strongest correlation is with the fibre size of the non-woven, and therefore also surface area, with fine separators absorbing the most ammonia.        “Samples of the PP fine fibre spunbond were also prepared with two different graft levels, and their ammonia absorption measured (see table 2). These results show that the ammonia absorption is independent of the total amount of acrylic acid grafted onto the polymer backbone, and is further evidence that it is the base non woven which controls the degree of ammonia absorption.”        